1. Field of the Invention
The present invention relates to a dropout compensator for a trisequential color video recording and playback system. In such a system the three color signals are recorded sequentially and played back within three consecutive lines whereas the Y high information is played back within each line. A signal having a dropout is automatically replaced by a signal scanned three lines before, which signal includes the same color information as the replaced signal.
2. Description of Prior Art
In video recording, how to compensate for defects in signals like dropouts has always been a problem. Such defects can originate from the recording medium itself, from the scanning means and/or from inaccurate head tracking. Dropout compensators are well known in the art and are used to provide information as a replacement for information which is missing from the video signal. In principle such a compensator works as follows.
When a modulated recorded video signal is played back, it is divided into two substantially identical components, one of the components being delayed with respect to the other component by an interval of time that is substantially equal to the time required for one horizontal scan period. Under normal conditions, when no defect occurs, the undelayed component is utilized and the delayed component is suppressed. When a defect in the signal such as a dropout is detected by a detector, a switch is operated which releases the delayed signal and cuts off the undelayed signal for the time the defect is in existence. Because a video signal during any given scan is largely redundant over the adjacent scan, the occasional and random repetition of a horizontal line of limited portions of such a line will generally go unnoticed by the viewer of a television screen for instance. This applies to both black and white video signals and color video signals if more than one color information is available during one line scan period.
In a trisequential color video system during each line scan period only one color information is produced, as will be clear from the following. When the first line includes the "red" information, the second line includes the "green" information and the third line contains the "blue" information. In the case of the foregoing method for dropout compensation, a "red" line signal including a defect would be replaced by a "blue" line signal, a "green" line signal by a "red" line signal and a "blue" line signal by a "green" line signal.
A dropout compensator disclosed in Germany Printed Application (DAS) No. 2,148,486 is applicable to a trisequential color video recording and playback system. Here, a compensating signal is used which is an aggregate signal derived from the color signals occurring during the last two line scan periods, calculated from the scan period within which a dropout occurs. When the dropout occurs within the "red" signal period, the said compensating signal consists of the aggregate "blue" and "green" signals.
It is clear to anyone skilled in the art that it is not possible to obtain good results with the above compensating method because dropouts which continue for a longish period, e.g. for as long as or longer than one line scan period, will be perceived by the viewer. This disadvantage is very serious, when at least one of the aggregate color signals has a high color intensity.
An object of the present invention is to overcome the disadvantages of conventional dropout compensators. A further object of the invention is to provide a highly effective method of compensating for dropouts in a trisequential color video recording and playback system. Yet another object of the invention is the provision of an inexpensive dropout compensator circuit.
An even further object of the invention is to achieve effective dropout compensation for color and luminance signal up to three line scan periods. The novel device according to this invention employs new and improved means for sensing the occurrence of a dropout, and new and improved means for switching from one signal to another signal during the existence of the abnormal condition without creating visually noticeable or objectionable alterations in the picture.
The device of the present invention is applicable to any trisequential color video system. Such a system may for example employ a magnetic tape, a magnetic disc, a piezoelectrical scannable disc or an optically scannable disc. In such recording and playback systems the use of color video signals brought into trisequential form is very advantageous because the usable bandwidth range of the recording medium is fully utilized. These and other features and advanatages of the present invention will become readily apparent from the following detailed description of one operative embodiment of the present invention, particularly when read in connection with the accompanying drawings.